Bit streams are used to represent the outputs of microphones, cameras, and a variety of other transducers. Bit streams are the input and output forms of computers. When the origin of the message of interest is distant from the user, then a means of transporting the bit streams is required. Commonly, this means is optical or electromagnetic. The means can be guided (fibers, cables, pairs of wire), or it can be unguided (radiation). Radiation makes use of carriers consisting of sinusoidal waves. Modulation of the sinusoidal carrier enables the transmission of the bit stream(s).
A simple form of a feature of a modulated carrier is its presence. That is, in a simple case, the presence of a sinusoid or the presence of optical energy (light) represents a binary 1, for example. The absence of these represents a binary 0. This is a standard means of transmitting bit streams over glass fibers and of recovering bit streams from compact disk recordings, for example. In radio communications, this simple form is called on-off amplitude modulation or on-off keying (OOK). In this case, only one bit stream is sent per carrier.
The amplitude, frequency or phase and combinations thereof of a sinusoidal carrier can be modulated to distinguish one bit from the other or a group of bits from another group. A variety of well known modulation schemes are treated in the literature. Types of modulation are characterized by bandwidth efficiency and power efficiency. Bandwidth efficiency relates to the band of frequency spectrum occupied by the modulated carrier for a given rate of bit transmission (bit rate in bits per second). Power efficiency relates to the probability of any bit received in error, or bit error ratio (BER), as a function of receiver input signal to noise ratio (SNR).
Conventional means of bit stream transmission do not consider use of multiple co-channel carriers to transmit one or more bit streams simultaneously. In this manner, spectrum can be reused thereby improving bandwidth efficiency. Modification of a bit by the inclusion of a recognizable feature is not treated as a means of increasing data throughput within a prescribed band of frequency. Theoretical comparisons of the types of carrier modulation rarely consider the complexity of the transmitter circuitry needed to generate and radiate the modulated carrier and the receiver circuitry required to recover the bit stream from the carrier. Therefore, because of the cost and availability of spectrum, and because of the cost associated with complexity with a small number of units, there is a need to define and develop data transmission systems which are simple, robust and bandwidth efficient.
U.S. Pat. Nos. 4,859,958 and 4,992,747, invented by Glen A. Myers, the inventor of the present invention, are each incorporated by reference in the present application as though fully set forth herein. In these patents, a means for demodulating all of several co-channel FM carriers is described.
U.S. Pat. No. 5,038,115, co-invented by the inventor of the present invention, is also incorporated by reference in the present application as though fully set forth herein. In this patent, phase tracking of input terminal signals is described. In one embodiment of the phase tracking circuit of U.S. Pat. No. 5,038,115, a phase tracking circuit makes use of two phase-locked loops electrically connected in a feed forward manner.
U.S. Pat. No. 5,329,242 invented by the inventor of the present invention, is also incorporated by reference as though fully set forth herein. In this patent, demodulating a frequency modulated signal using the time intervals between zero crossings of a received carrier signal is described. Averaging and mapping techniques are used to improve estimates of the message signal.
U.S. Pat. Nos. 5,541,959 and 5,570,395, invented by the inventor of the present invention, are also incorporated by reference as though fully set forth herein. These patents describe, analytically and geometrically, the effect of adding two sinusoids of different frequency.
U.S. Pat. No. 5,606,581, invented by the inventor of the present invention is also incorporated by reference as though fully set forth herein. This application described a method and apparatus for creating a replica of a dominant carrier.
U.S. Pat. No. 5,554,955, invented by the inventor of the present invention, is also incorporated by reference as though fully set forth herein. This patent describes method and apparatus for removing the effects of co-channel interference from the message on a dominant frequency modulated carrier and for recovering the message from each of two co-channel carriers.
U.S. patent application Ser. No. 08/705,721 by the inventor of the present invention is also incorporated by reference as though fully set forth herein. This application describes method and apparatus for recovering the independent bit streams from each of two co-channel frequency modulated carriers.